proper check of "db" property of ActiveDataProvider.

framework/yii/data/ActiveDataProvider.php

@@ -86,9 +86,9 @@ class ActiveDataProvider extends BaseDataProvider
 		parent::init();
 		if (is_string($this->db)) {
 			$this->db = Yii::$app->getComponent($this->db);
-			if (!$this->db instanceof Connection) {
-				throw new InvalidConfigException('The "db" property must be a valid DB Connection application component.');
-			}
+		}
+		if ($this->db === null) {
+			throw new InvalidConfigException('The "db" property must be a valid DB Connection application component.');
 		}
 	}